hot and cold breath
We all know how to breathe hot air ("hahhh") and cold air ("whooo") — but *what* is making it hot or cold?
Jason: I have wondered that myself in the past... I would assume: 1) speed of air, 2) surface area of mouth opening, and 3) whether the air comes from the bronchial tubes or mouth???
Barry: But the air invariably comes from the lungs, and even when you "whooo" slowly it's still cool. A puzzlement!
Jason: I think it is primarily the surface area of the opening and cavity. With the hahhh you have a LOT more, and that is all at 98.6
Barry: Compelling: something about the ratio of 98.6º surface area to the mass of air flowing past it. But then that begs the question: what temperature was the air when it was *in* your body?
Jason: All 98.6. But the reason it would be cooler when quickly pushed through with a whoooo, is same effect of a "swamp" cooler (or evaporative cooler)
Barry: On reflection, I think that has to be it.
Kelly: It acts on the same principle as an air conditioner, when a gas expands it cools. By pursing your lips you can compress the air by "lip resistance" and releasing it causes it to expand. By exhaling there is no pressurization.
Carl: The problem with that explanation is that the pressurization would increase the temperature as much as the depressurization decreases it.
I've always assumed it has to do with velocity (or, more accurately, convection). The air is lower than body temperature either way because it is only exposed to the inside of your body for a second or two, but it is nonetheless higher than the ambient air. So when it comes out with negligible velocity, you can sense that it is hotter than ambient temperature, but when it comes out at a higher speed, it is transfers a significant enough amount of energy away from your skin (being of a lower temperature than your skin) that it seems cooler than ambient air.
This is vaguely similar to the reason that 75° water feels cooler than 75° air: it's coefficient of convection is higher. But in the situation at hand, you're increasing the coefficient by adjusting the velocity of the fluid (this is not a technically correct statement) rather than by substituting a more convective fluid.
Kelly: But if it were merely convection this would only have an effect on the skin as consistent with "wind chill". This effect would not have an effect on something without moisture, such as a window. Even if you huff quickly it will fog or purse your lips and blow slowly it will not.
As I've been contemplating the water temperature vs the air temperature on the body could it be that water is a greater great heat sink and has a greater capacity? Possibly?
Carl: Kelly, you're right that my argument essentially boils down to "wind chill." Convection affects any body, though, with or without moisture; it's just a matter of heat transfer and the 3rd law of thermodynamics. I'm not sure I understand your point about fog on a window, but I'll readily admit I don't know why the moisture in your breath would be more likely to condense on a window when blown "open mouth," but not "closed." My best guess (off the top of my head) would still be velocity. Even if you blow fast "open" and slow "closed," I imagine the latter would still be considerably faster than the former because of the extreme effect of throttling. If that's the case, then the moisture simply wouldn't have time to settle on the glass. (I recognize that may be faulty, though.)
And when you say that water is a greater heat sink, that's essentially the same as saying that it has a higher coefficient of convection, as I understand it.
Kelly: Would moisture condense in the mouth as an effect of greater velocity in the mouth cavity? This is quite a quandary...
Carl: you've gone beyond even my pretended knowledge there. I guess greater pressure would make it condense... I think.
Kelly: Okay, being the scientist that I am, I had to perform an experiment to figure this out.
Equipment: Calibrated digital thermometer with a K-Type thermocouple.
Control was ambient air temp. 79.8° F
First experiment: pursed lips, quick blow: Temp increase 6.1° F. Open mouth "huff": Temp increase 10.9° F.
Second experiment in a refrigerator. Temp 36° F
Pursed lips, quick blew: Temp increase 10.3°F. Open mouth "huff": 15.2° F.
Conclusion: Ambient air is taken in and incorporated more efficiently with a thin, fast paced stream of air more than a wide slow stream of air. In addition, the temperature differential (delta T) between refrigerated air is heated and caused a greater effect.
Any other observations are welcome. Y'all have a great weekend! Tschuss!
PS: In addition, I moved the thermocouple closer to my pursed lips and the temperature differential was greater (less air infusion).
Carl: wow... impressed. I'm just an armchair engineer.
Kelly: And yes, Carl, the feel on our skin would thus be very influenced by ambient air at a higher velocity, thus the "wind chill effect". Therefore the award goes to us both! Cheers!
Jason: I have wondered that myself in the past... I would assume: 1) speed of air, 2) surface area of mouth opening, and 3) whether the air comes from the bronchial tubes or mouth???
Barry: But the air invariably comes from the lungs, and even when you "whooo" slowly it's still cool. A puzzlement!
Jason: I think it is primarily the surface area of the opening and cavity. With the hahhh you have a LOT more, and that is all at 98.6
Barry: Compelling: something about the ratio of 98.6º surface area to the mass of air flowing past it. But then that begs the question: what temperature was the air when it was *in* your body?
Jason: All 98.6. But the reason it would be cooler when quickly pushed through with a whoooo, is same effect of a "swamp" cooler (or evaporative cooler)
Barry: On reflection, I think that has to be it.
Kelly: It acts on the same principle as an air conditioner, when a gas expands it cools. By pursing your lips you can compress the air by "lip resistance" and releasing it causes it to expand. By exhaling there is no pressurization.
Carl: The problem with that explanation is that the pressurization would increase the temperature as much as the depressurization decreases it.
I've always assumed it has to do with velocity (or, more accurately, convection). The air is lower than body temperature either way because it is only exposed to the inside of your body for a second or two, but it is nonetheless higher than the ambient air. So when it comes out with negligible velocity, you can sense that it is hotter than ambient temperature, but when it comes out at a higher speed, it is transfers a significant enough amount of energy away from your skin (being of a lower temperature than your skin) that it seems cooler than ambient air.
This is vaguely similar to the reason that 75° water feels cooler than 75° air: it's coefficient of convection is higher. But in the situation at hand, you're increasing the coefficient by adjusting the velocity of the fluid (this is not a technically correct statement) rather than by substituting a more convective fluid.
Kelly: But if it were merely convection this would only have an effect on the skin as consistent with "wind chill". This effect would not have an effect on something without moisture, such as a window. Even if you huff quickly it will fog or purse your lips and blow slowly it will not.
As I've been contemplating the water temperature vs the air temperature on the body could it be that water is a greater great heat sink and has a greater capacity? Possibly?
Carl: Kelly, you're right that my argument essentially boils down to "wind chill." Convection affects any body, though, with or without moisture; it's just a matter of heat transfer and the 3rd law of thermodynamics. I'm not sure I understand your point about fog on a window, but I'll readily admit I don't know why the moisture in your breath would be more likely to condense on a window when blown "open mouth," but not "closed." My best guess (off the top of my head) would still be velocity. Even if you blow fast "open" and slow "closed," I imagine the latter would still be considerably faster than the former because of the extreme effect of throttling. If that's the case, then the moisture simply wouldn't have time to settle on the glass. (I recognize that may be faulty, though.)
And when you say that water is a greater heat sink, that's essentially the same as saying that it has a higher coefficient of convection, as I understand it.
Kelly: Would moisture condense in the mouth as an effect of greater velocity in the mouth cavity? This is quite a quandary...
Carl: you've gone beyond even my pretended knowledge there. I guess greater pressure would make it condense... I think.
Kelly: Okay, being the scientist that I am, I had to perform an experiment to figure this out.
Equipment: Calibrated digital thermometer with a K-Type thermocouple.
Control was ambient air temp. 79.8° F
First experiment: pursed lips, quick blow: Temp increase 6.1° F. Open mouth "huff": Temp increase 10.9° F.
Second experiment in a refrigerator. Temp 36° F
Pursed lips, quick blew: Temp increase 10.3°F. Open mouth "huff": 15.2° F.
Conclusion: Ambient air is taken in and incorporated more efficiently with a thin, fast paced stream of air more than a wide slow stream of air. In addition, the temperature differential (delta T) between refrigerated air is heated and caused a greater effect.
Any other observations are welcome. Y'all have a great weekend! Tschuss!
PS: In addition, I moved the thermocouple closer to my pursed lips and the temperature differential was greater (less air infusion).
Carl: wow... impressed. I'm just an armchair engineer.
Kelly: And yes, Carl, the feel on our skin would thus be very influenced by ambient air at a higher velocity, thus the "wind chill effect". Therefore the award goes to us both! Cheers!
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